1. Field of the Invention
The present invention relates to a vacuum contact type printing machine and a method of contact therefor employed in a process of reproduction. More specifically, the present invention relates to a large vacuum contact type printing machine and a method of contact therefor.
2. Description of the Related Art
A printing machine of interest to the present invention is disclosed in U.S Pat. No. 4,484,813. FIG. 8 shows a main portion of the printing machine shown in this patent. Referring to FIG. 8, a conventional printing machine comprises a printing table 102 holding a flexible blanket 126 on a blanket support plate 122 and an upper frame 103 which can be opened/closed in relation to the printing table 102. The upper frame 103 comprises a transparent plate 105 arranged on a lower surface of an edge frame 104. An airtight seal 114 is provided between the flexible blanket 126 and the transparent plate 105, whereby an exposure chamber A which is air tight is formed. The printing machine further comprises a vacuum pump VP for evacuating the exposure chamber A and a blanket attracting apparatus 130 which evacuates the air in a space B between the flexible blanket 126 and the blanket support plate 122. The space B may be maintained at atmospheric pressure as needed. When the air in the space B is evacuated, the flexible sheet 126 is brought into sealed contact with the blanket support plate 122.
An original film S and a photosensitive material P are held in contact between the flexible sheet 126 and the transparent plate 105 in the following manner. At first, the flexible blanket 126 is attracted to the blanket support plate 122 by means of the blanket attracting apparatus 130. In this state, the pressure in the exposure chamber A is reduced by the vacuum pump VP. Thereafter, the pressure in the space B is kept at atmospheric pressure.
The flexible blanket 126 is attracted to the blanket support plate 122 until the pressure in the exposure chamber A is reduced to some extent. Thereafter, the operation of the blanket attracting apparatus 130 is stopped and the pressure in the space B between the flexible blanket 126 and the blanket support plate 122 is kept at atmospheric pressure. Consequently, as shown by the two dotted chain lines in the figure, the flexible blanket is lifted by the atmospheric pressure in space B. Consequently, the original film S is brought into close contact with the photosensitive material P as shown by the two dotted chain lines in FIG. 8.
The conventional printing machine is structured as described above. It is adapted such that the flexible blanket 126 and the transparent plate 105 are brought into close contact gradually from the central portion to the peripheral portions. Consequently, any entrapped air which is likely to be present between the original film S and the photosensitive material P is readily removed. When attraction is done by the blanket attracting apparatus 130 or when the pressure in the space B is to be maintained at the atmospheric pressure, air is allowed to flow through communicating holes C.sub.1.
The conventional printing machine having the above described structure has the following disadvantages:
(1) Since the atmospheric pressure works uniformly on the lower surface of the flexible blanket 126 through the communicating holes C.sub.1, the flexible blanket 126 is not always brought into contact with the transparent plate 105 from the central portion to the peripheral portions.
(2) Especially in a printing machine having a large printing table, the above described disadvantage (1) becomes more conspicuous and air is more likely to be entrapped in the central portion. Therefore, it is difficult to realize satisfactory vacuum contact at high speed.